Session Info

For disputes and approvals, we need access to information about validator sets from prior sessions. We also often want easy access to the same information about the current session's validator set. This module aggregates and stores this information in a rolling window while providing easy APIs for access.

Storage

Helper structs:

#![allow(unused)]
fn main() {
struct SessionInfo {
    /// Validators in canonical ordering.
    ///
    /// NOTE: There might be more authorities in the current session, than `validators` participating
    /// in parachain consensus. See
    /// [`max_validators`](https://github.com/paritytech/polkadot/blob/a52dca2be7840b23c19c153cf7e110b1e3e475f8/runtime/parachains/src/configuration.rs#L148).
    ///
    /// `SessionInfo::validators` will be limited to `max_validators` when set.
    validators: Vec<ValidatorId>,
    /// Validators' authority discovery keys for the session in canonical ordering.
    ///
    /// NOTE: The first `validators.len()` entries will match the corresponding validators in
    /// `validators`, afterwards any remaining authorities can be found. This is any authorities not
    /// participating in parachain consensus - see
    /// [`max_validators`](https://github.com/paritytech/polkadot/blob/a52dca2be7840b23c19c153cf7e110b1e3e475f8/runtime/parachains/src/configuration.rs#L148)
    #[cfg_attr(feature = "std", ignore_malloc_size_of = "outside type")]
    discovery_keys: Vec<AuthorityDiscoveryId>,
    /// The assignment keys for validators.
    ///
    /// NOTE: There might be more authorities in the current session, than validators participating
    /// in parachain consensus. See
    /// [`max_validators`](https://github.com/paritytech/polkadot/blob/a52dca2be7840b23c19c153cf7e110b1e3e475f8/runtime/parachains/src/configuration.rs#L148).
    ///
    /// Therefore:
    /// ```ignore
    ///		assignment_keys.len() == validators.len() && validators.len() <= discovery_keys.len()
    ///	```
    assignment_keys: Vec<AssignmentId>,
    /// Validators in shuffled ordering - these are the validator groups as produced
    /// by the `Scheduler` module for the session and are typically referred to by
    /// `GroupIndex`.
    validator_groups: Vec<Vec<ValidatorIndex>>,
    /// The number of availability cores used by the protocol during this session.
    n_cores: u32,
    /// The zeroth delay tranche width.
    zeroth_delay_tranche_width: u32,
    /// The number of samples we do of `relay_vrf_modulo`.
    relay_vrf_modulo_samples: u32,
    /// The number of delay tranches in total.
    n_delay_tranches: u32,
    /// How many slots (BABE / SASSAFRAS) must pass before an assignment is considered a
    /// no-show.
    no_show_slots: u32,
    /// The number of validators needed to approve a block.
    needed_approvals: u32,
}
}

Storage Layout:

#![allow(unused)]
fn main() {
/// The earliest session for which previous session info is stored.
EarliestStoredSession: SessionIndex,
/// Session information. Should have an entry from `EarliestStoredSession..=CurrentSessionIndex`
Sessions: map SessionIndex => Option<SessionInfo>,
}

Session Change

1. Update EarliestStoredSession based on config.dispute_period and remove all entries from Sessions from the previous value up to the new value.
2. Create a new entry in Sessions with information about the current session. Use shared::ActiveValidators to determine the indices into the broader validator sets (validation, assignment, discovery) which are actually used for parachain validation. Only these validators should appear in the SessionInfo.

Routines

• EarliestStoredSession::<T>::get() -> SessionIndex: Yields the earliest session for which we have information stored.
• Sessions::<T>::get(session: SessionIndex) -> Option<SessionInfo>: Yields the session info for the given session, if stored.